The advent of highly mobile tank warfare at the
start of World War 2 prompted a search for ways of destroying tanks
from the air, with variable success. There was a preference in some
quarters for using fighter-bombers armed with rockets or bombs, but
while these were effective in general ground attack, including
disrupting supplies to armoured units, they proved largely ineffective
in directly knocking out tanks for the reasons spelled out in 'Flying
Guns: WW2':

" The ineffectiveness of air attack against
tanks should have caused no surprise because the weapons available to
the fighter-bombers were not suitable for destroying them. Put simply,
the heavy machine guns and 20 mm cannon were capable of hitting the
tanks easily enough, but insufficiently powerful to damage them, except
occasionally by chance. The RPs and bombs used were certainly capable
of destroying the tanks but were too inaccurate to hit them, except
occasionally by chance."

Experience showed that the best way of knocking
out tanks was to use a cannon powerful enough to penetrate the armour.
This article examines the weapons used and takes a retrospective look
at an 'ideal' airborne anti-tank armament.

The Guns

A wide variety of airborne anti-tank guns was
fielded by three nations: the USSR, Germany and the UK. The USA
developed one, but did not use it. This section describes their
characteristics, but the armour penetration figures need to be regarded
with caution because of the varying standards of measurement used in
different countries. The variations not only include the usual
parameters of striking distance and angle, but also the quality of the
armour and the definition of 'penetration'. Also, some figures resulted
from ground tests, others allowed for the extra velocity imparted by
the speed of the aircraft.

The USSR fielded two guns in the anti-armour
role, and tested a third. The 23 mm VYa and 37 mm NS-37 were developed
specifically for aircraft and used in both the air combat and anti-tank
roles. The NS-45 was a version of the NS-37, with the cartridge case
necked out to 45 mm. It was tested in air combat and proposed for
various anti-tank aircraft, but not adopted.

The VYa was a gas-operated, belt-fed gun (a
scaled-up 12.7 mm Berezin) chambered for a powerful 23 x 152B cartridge.
It is credited with penetrating 30 mm at 100 m and 25 mm at 400 m (striking
angle unspecified). This was adequate against light tanks but not
enough to reliably penetrate medium tanks.

The NS-37, unusually for the USSR, used a
short-recoil mechanism and was belt-fed. It was a particularly slim
weapon, with a compact mechanism, suitable for fitting between the
banks of a liquid-cooled vee-engine to fire through the hollow
propeller hub. The powerful 37 x 195 ammunition is quoted as penetrating
48 mm / 500 m / 90 degrees, enough to pose a threat to the side or rear
armour of virtually any tank. The 45 x 185 cartridge of the NS-45
increased this to around 58 mm.

All of the Soviet guns used conventional
full-calibre steel shot, fitted with a windshield to improve the
ballistic coefficient. This is rather surprising, as they used the much
more effective tungsten-cored projectiles in ground anti-tank guns.

Germany fielded several different guns in the
anti-tank role. The first was the 30 mm MK 101, which used powerful
30 x 184B ammunition. This was later supplanted by the MK 103, which used
an electric-primed version of the same cartridge, with the same
performance. The MK 101 used a short-recoil design, and in its
anti-armour role was fed by a 30-round drum. The more compact
and faster-firing MK 103 used a hybrid gas+recoil system and was
belt-fed. Various AP rounds were used, but the most effective was the Hartkernmunition,
which had a penetrating core of tungsten carbide sheathed in a
light-alloy shell with a sharply-pointed profile. This could penetrate
75-90 mm / 300 m / 90 degrees (depending on the type of armour),
dropping to 42-52 mm when impacting at 60 degrees.

The remaining German guns were all adaptations of
ground guns. The first was the 37 mm BK 3,7, a modified version of the
FlaK 18 AA gun firing the same 37 x 263B ammunition. This meant that it
was bulky, heavy and slow-firing by comparison with the NS-37, for
example. It also remained clip-fed, with a maximum capacity of just 12
rounds. It mainly fired Hartkernmunition ammo, capable of penetrating up to 140 mm /
100 m / 90 degrees although this was halved at a striking angle of 60 degrees.

The remaining German guns were adaptations of
ground anti-tank guns with long-recoil mechanisms, and saw little use.
These were the BK 5 (also briefly used for air combat in the
anti-bomber role) which was a PaK 38 fitted with an autoloader and a
22-round magazine for its 50 x 419R ammunition. It is unclear whether
this saw action in the anti-tank role. More famously, the PaK 40, which
fired massive 75 x 714R cartridges, was used in two versions; a
semi-automatic one with a 10-round vertical magazine, and the
fully-automatic BK 7,5 with a 12-round rotary magazine. This could
penetrate 132 mm / 500 m / 90 degrees (104 mm / 500 m / 60 degrees)
when used on the ground, so was clearly capable of dealing with the
toughest tank. Both BK 5 and BK 7,5 fired the normal APCHE projectiles,
which were more destructive and reliable in their penetration than the Hartkernmunition rounds, being less affected by unfavourable striking angles or add-on armour plates.

The British fielded only one airborne gun in the
anti-tank role: the Vickers Class S. This was designed around the naval
40 x 158R AA case, with special armour-piercing loadings. As such, it was
much less powerful than the Army's 2 pdr anti-tank gun, but the attack
speed of the aircraft helped to provide a penetration quoted as 50-55 mm
(range and striking angle not specified). This was a long-recoil gun
which was fed by a 15-round drum (a sixteenth round could be carried in
the chamber). A 30-round drum and a belt-fed version were developed but
not adopted. The same fate befell an interesting adaptation to use the Littlejohn squeezebore shot. The AP projectiles fired were full-calibre
steel shot, without even a ballistic cap.

The British developed other weapons for this purpose. The 6 pdr (57 mm) Molins gun was intended for this role, but used only by Coastal
Command. The 47mm Class P gun was not ready until after the war.

The USA did fit one 37 mm gun to its aircraft -
the M4. However, this fired low-powered 37 x 145R ammunition and was not
very effective against tanks, being intended for air combat. Much more
impressive was the M9, which was a belt-fed development of the Army's
M1 long-recoil AA gun and fired a powerful 37 x 223SR cartridge. Special
AP loadings were developed for this gun but it was never used in action.

Airborne anti-tank guns

Designation

Cartridge

Projectile
Weight grams

Muzzle Velocity
m/s

Muzzle Energy
joules

Gun weight kg

Gun length cm

Rate of Fire rpm

VYa

23x152B

200

880

77,400

68

214

550

NS-37

37x195

760

880

294,000

170

341

250

MK 101

30x184B

355

940

157,000

180

259

230

MK 103

30x184B

355

940

157,000

141

235

360

BK 3,7

37x263B

405

1,140

263,000

295

363

160

BK 5

50x419R

2,060

835

718,000

540

434

45

BK 7,5

75x714R

6,800

790

2,120,000

705

610

30

Class S

40x158R

1,130

615

214,000

134

297

100

M9

37x223SR

753

930

326,000

181

264

140

Cartridge Illustrations

The following illustrations show most of the
cartridges listed above (except for the 37 x 223SR which did not see service).

Above: cartridges of up to 40mm used in ground attack, starting with the .50 Browning
(12.7x99) and 20mm Hispano (20x110) to give an idea of the relative scale and
power.

Looking at the targets which the guns had to deal
with, it would seem that for the weapon to be useful against any tank
on the battlefield, the minimum performance should be a penetration of
100 mm / 300 m / 90 degrees, and 60 mm / 300 m / 60 degrees. If
conventional steel shot were used, then a gun as big as the BK 5 would
be required to give such performance. This argues in favour of using
tungsten-cored shot for this purpose (despite some disadvantages), in
order to keep the gun relatively light and compact. The required
performance is a bit better than the MK 101/103 achieved firing
tungsten-cored shot, which in 30 mm calibre indicates that a muzzle
energy of around 200,000 joules would be sufficient. Interestingly, the
current 30x173 API loading for the GAU-8/A used in the A-10 tankbuster
develops 207,000 joules, and also uses a Hartkernmunition shot, although with a depleted uranium rather than tungsten carbide core.

In WW2 the 30 mm guns were criticised for the
limited behind-armour effect of their small penetrating cores (less of
a problem for the GAU-8/A: an advantage of DU, apart from 10-15% better
penetration, is that it has an incendiary effect after penetrating).
This argues in favour of a 37-40 mm gun to provide greater
effectiveness. A larger calibre would also provide more effective HE
shells for use against softer targets.

Studying the figures, it looks as if a a muzzle
energy of around 250,000 joules would be needed in 37 mm calibre. The
ideal loading would use a heavier projectile than the rather light 405 g
shot of the BK 3,7 in the interests of better performance at long range
(the weight of the GAU-8/A's 30 mm DU projectile is 425 g). On the other
hand, a high velocity is also desirable to flatten the trajectory and
minimise the flight time, thereby increasing the hit probability. A
reasonable compromise would be 500 g at 1,000 m/s, which works out
nicely at 250,000 joules. Alternatives would be 555 g at 950 m/s, or
615 g at 900 m/s. This is actually less powerful than all of the service
37 mm AT guns, so we can use a smaller and lighter cartridge case;
similar to the NS-37's, but a bit slimmer.

What of the gun? Clearly it should be belt fed,
in fact ideally with a double-belt feed so that the pilot could switch
between AP and HE ammunition as required; that would be cheating a bit
as double-belt feeds didn't enter service until after WW2, but it was a
straightforward engineering task so could have been introduced earlier
if the need had been identified. The choice of mechanism is
interesting: the best performer in terms of rate of fire was the NS-37,
but that used a short-recoil mechanism which imposed considerable
stresses on the aircraft carrying it (the initial aircraft
installation, mounted to the engine, resulted in a cracked engine
block). A long-recoil design like the Vickers Class S had only half the
rate of fire, but the recoil was moderated into a smooth push which was
much easier to manage. This not only meant fewer installation problems,
but also reduced the disturbance to the aircraft, making it easier to
keep the sights on target during a firing pass. A weight of around 150
kg, a length of 3 m and a RoF of 150 rpm would seem reasonable targets.
Ammunition capacity should be at least 30 rounds per belt; normally one
belt of AP and one of HE, although that could be altered depending on
the circumstances.

The Gun Carriers: what of the aircraft?

There were various possible locations for heavy
cannon, but they all boiled down to two basic types - wing or fuselage
mountings. Wing mountings had several disadvantages. First, they
suffered the usual problems with this location of harmonisation; that
is, the guns had to be angled inwards to coincide with the sight line,
and this could only be for a specified distance. At much shorter or
longer distances, the projectiles would not strike where the sights
were aimed. Incidentally, it is worth mentioning that with all guns,
wherever mounted, there was also a vertical harmonisation issue, in
that projectiles followed a curved trajectory so at much shorter or
longer distances they would strike above or below the aiming point.
However, this would be much less of a problem with a high muzzle
velocity. Other disadvantages of wing mounting peculiar to heavy cannon
were, obviously, that the weight and drag penalties were much greater
than with one fuselage-mounted gun, the wings were more flexible not
just in flight but also under recoil (which affected accuracy) and the
plane could be moved from side-to-side by the recoil if the guns fired
at different moments, further affecting accuracy. The guns were also
mounted under the plane's centre of gravity, which meant that recoil
pushed the nose down on firing.

All of this added up to less accuracy and lower
aircraft performance with wing-mounted cannon. This is significant
because most single-engined aircraft fitted with large cannon had no
option but to fit them under the wings, as few vee-engines, and no
radials, were compatible with the engine-mounting of the gun to fire through a hollow propeller hub. In
theory, a gun could be mounted under the belly of the plane and
synchronised to fire through the propeller disk, but in practice this
became more problematic as the size of the cartridge case increased
because of the variations in the burning time of the propellant (the
Luftwaffe considered but rejected such an installation of the MK 103 in
the Fw 190 for this reason).

Of course, none of these problems bothered
twin-engined aircraft, which were able to employ a rigid central
mounting in or under the fuselage, directly under the sights, to the
great benefit of accuracy. They also usually had a much better forwards
and downwards view (important for a ground attack plane), although
blocked to the side by the engines. However, most twins were much
bigger than singles, making them much bigger targets for ground fire.
Attempts to use Ju 88s in the anti-tank role led to catastrophic loss
rates. The only exception was the Hs 129, a very compact plane.
However, while that was probably the most effective of all the
anti-tank aircraft, it had a low performance and was relatively
helpless against enemy fighters, not even having the benefit of a rear
gunner.

The table below lists some aircraft which were
used, or potentially useful, as anti-tank planes. Wing area is an
indicator of apparent size, and therefore the risk of being hit by
ground fire (although for a given wing area, twin-engined planes
usually presented a bigger target). Wing loading is a double-edged
sword: a low wing loading was useful for a good short-field performance
and low-speed manoeuvrability, but a smaller wing was less disturbed by
low-level air turbulence and therefore (other things being equal) was
likely to provide a more stable gun platform for ground attack. The
power loading (as with the wing loading, taken at maximum loaded
weight) gives an indicator of performance.

TYPE

EMPTY WT KG

LOADED WT KG

WING AREA M2

WING LOADING
KG/M2

POWER HP

POWER LOADING
HP/TONNE

Yak-9T

2,300

3,000

17.2

174

1,180

393

Bell P-39D

2,500

3,500

19.8

177

1,150

329

Hurricane II

2,650

3,650

23.9

153

1,260

345

Fw 109F-3

3,200

4,900

18.3

268

1,700

345

Hawker Typhoon

4,000

6,350

25.9

245

2,260

356

P-47D

4,500

6,800

27.9

244

2,300

338

Il-2M-3

4,600

6,200

38.5

161

1,750

282

Ju-87G

4,500

6,500

31.9

204

1,400

215

Hs 129B-2

3,800

5,250

29.0

181

2 x 700

267

Whirlwind

3,800

5,200

23.2

224

2 x 865

340

Gloster F9/37

4,000

5,250

35.9

146

2 x 1,000

380

P-38F

5,500

9,800

30.4

322

2 x 1,425

291

Beaufighter TFX

7,000

11,500

46.3

248

2 x 1,770

308

Mosquito FB.XVIII

7,000

10,000

42.2

237

2 x 1,635

327

Of the single-engined aircraft, the Yak-9T and
P-39 were not intended as ground attack planes, but they had the
potential as their layouts permitted the carrying of heavy guns.

The Whirlwind was considered for mounting a 40 mm
S gun in the nose, and as it was no bigger than the Hs 129 it might
have made a good tankbuster - if it weren't for those liquid-cooled
engines. An alternative engine available at the time was the Bristol
Taurus, a slim, two-row, air-cooled radial which weighed about the same
but was more powerful. The potential was clearly there. One
twin-engined fighter was designed around this engine; the Gloster
F9/37. It was built and tested, and proved to have a high performance
and excellent handling, but was one of those unlucky planes which was
not ordered despite its qualities. It would presumably have weighed
quite a lot more in ground-attack trim, with armour and a heavy gun,
but had the advantage over the Whirlwind of a forward cockpit, giving
excellent forwards and downwards visibility. Of the other twins, the
P-38 and Mosquito (the 'Tsetse', with a 57 mm Molins gun) were also
handicapped by liquid-cooled engines. The Tsetse was originally
conceived as an anti-tank plane, but not used for that purpose. The gun
was accurate and effective, so it was the plane's vulnerability which
must have counted against it. The Beaufighter had air-cooled radials
and a reputation for toughness, but the sheer size of this beast seems
to have ruled it out of consideration for the anti-tank role.

The Ilyushin Il-2 and Ju 87G were effective
armoured attack planes. Although equipped with vee-engines, they were
handicapped by installations which did not permit an engine-mounted
gun, so they had to carry their armament in or under the wings (in the
case of the Ju 87G, the BK 3,7 was far too large to fit in an engine
mounting anyway). This gave them poor performance by comparison with
fighter-bombers, and they were vulnerable to fighters, although they
did have rear gunners to keep a look-out.

Our search for an anti-tank aircraft, which
combines carrying a powerful cannon with enough performance and agility
to defend itself against fighters, therefore results in two main
options: a small, fast twin like the hypothetical Taurus Whirlwind, or
a single-engined plane able to take a nose-mounted cannon, which leaves
us with the Soviet Yak-9 (and the inferior LaGG) and the American P-39
(and P-63 development).

The Yak-9T, with one NS-37 mounted on the engine
and a synchronised 12.7 mm Berezin, was a good fighter well able to look
after itself, but it wasn't perfect in the anti-tank role. The gun's
heavy recoil pushed the nose out of alignment, so only two or three
shots could be fired before the pilot had to correct the aim, and it
carried insufficient armour to protect its vulnerable engine cooling
system from ground fire (it took only a single bullet to penetrate the
radiator or coolant lines of a liquid-cooled engine to ensure that the
plane would not return to base). There was also room behind the engine
for only 32 rounds of cannon ammunition. Our 'ideal' gun would be a bit
lighter, and the ammunition would be slightly smaller, allowing weight
and space for more armour and ammunition (or alternatively an
additional Berezin). However, to provide the kind of comprehensive
armour protection carried by the Il-2 would probably have crippled the
performance.

The P-39 had in some ways a better layout than
the Yak-9. Locating the engine behind the pilot left the entire nose
clear for armament, which enabled the bulky M4 cannon (with 30 rounds)
plus a pair of .50 Brownings to be fitted. The later version of the
gun, the M10, had belt feed allowing no less than 58 rounds to be
carried in the later marks of P-63. One
prototype P-63D was fitted with the much more powerful 37 mm M9 cannon
described above, which could have had considerable anti-tank potential.
However, like the Yak-9 the plane was powered
by a liquid-cooled engine, which would have required substantial
additional armour to protect the cooling system from ground fire. The
synchronised .50s were also rather slow-firing, and the forward
position of the ammunition supply meant that the handling was affected
as the supply was used up.

Ground-attack planes were much better with
air-cooled (i.e. radial) engines, as they avoided all of the
vulnerabilities of the liquid cooling system and therefore needed less
comprehensive armouring, to the benefit of performance. In theory, the
P-39 layout could have been used with a radial engine although
providing adequate cooling would have been difficult. A better solution
technically could have been a twin-boom radial pusher, with the cockpit
mounted well forward (improving forwards and downwards visibility) and
the gun mechanisms and ammo supply behind the cockpit. The accompanying
HMGs also would not have needed to be synchronised. This layout would
have carried some structural weight and aerodynamic penalties, but
would have made an interesting alternative to the options available in
World War 2.